First, direct measurement
The method of directly measuring the aperture using two or three-point positioning is also the most commonly used aperture measurement method. According to the accuracy level, size and quantity of the aperture to be measured, general length measuring tools that can measure the aperture can be used, such as vernier calipers (see calipers), tool microscopes, universal length comparators, horizontal length gauges (see length measuring machines), horizontal optical gauges (see comparators) and pneumatic measuring instruments, etc.; special aperture measurement tools can also be used, such as internal diameter micrometers, internal diameter dial indicators and dial indicators, internal diameter micrometers, electronic plug gauges, and aperture gauges based on pneumatic, optical, electrical and other principles.

① Use a lever mechanism to measure the hole: commonly used in hand-held aperture measurement tools, such as internal diameter dial indicators, mechanical or electrical internal diameter micrometers, etc. The difference between the measured aperture size and the calibration ring gauge aperture is read from the dial indicator, and mechanical or electrical micrometer through a lever mechanism. The measuring range of the aperture of this type of hole measuring tool is generally 10 to 800 mm, among which the measuring accuracy of the internal diameter micrometer can reach 3 to 5 microns.
② Using the principle of the inclined wedge to measure the aperture: commonly used in hand-held aperture measuring tools. Among them, the internal diameter micrometer used to measure small holes can measure holes with a diameter as small as 0.5 mm. When the measured aperture compression probe moves the measuring rod with a cone, the aperture error can be read from the micrometer or micrometer. The three-point positioning method is suitable for measuring holes with a diameter of more than 3 mm. When the measuring rod rotates, the measuring rod moves forward by the fixed nut, and the cone with a spiral boss at the top of the measuring rod moves the three measuring heads outward to contact the measured hole. Read the measured aperture size from the scales on the fixed sleeve and the differential cylinder. This type of aperture measuring tool has a three-jaw internal diameter micrometer.
③ The seat-type aperture measuring instrument made of pneumatic, optical, electric, and other principles measures high-precision apertures, which must be carried out under constant temperature conditions close to 20°C. The range of aperture measurement of the light wave interference aperture measuring instrument is 1 to 50 mm, with an accuracy of ±0.5 microns.

Second, indirect measurement
First measure the function related to the aperture, and then convert the aperture size. There are mainly two methods: ① Using the principle of three points defining a circle, measure the coordinate values of any three points on the circumference of the measured hole, and then calculate the coefficients D, E, and F in the equation x2＋y2＋Dx＋Ey＋F＝0, and then calculate the measured aperture according to the calculation formula. This method is generally used in three-coordinate measuring machines with electronic computers;
② Use a roller with a known diameter to roll against the wall of the measured hole, measure the circumference of the measured hole, and then calculate the aperture. This method is suitable for measuring holes with a diameter greater than 500 mm and a continuous surface. The measuring tool that applies this method is called a large-diameter measuring instrument, which is also commonly used for measuring the outer diameter of large workpieces. (However, this measurement method is not very accurate. Some scholars have studied multi-roller measurement and computer simulation calculation methods to improve accuracy, but I think it has little practical value.)

Third, comprehensive measurement
It mainly uses smooth plug gauges to check whether the inner hole of the steel pipe or the mechanical workpiece is qualified by the pass-stop method. I will not explain it here.